Inductance coil



May 23, 1950 Original Filed Dec. 8, 1942 s. Y. vw-MTE. 2509;@49

INDUCTANCE COIL 4 Sheets-Sheet l S. Y., WHITE INDUCTANCE COIL May 239 1950 May 23, E95@ Y. val-MTE QQQQ [NDUCTANCE COIL Original Filed Dec. Se. 194.2 4 Sheets-Sheet 3 S5. Y. Vx/HTE INDUCTANCE COIL May 23, 1959 4 Sheets-Sheet 4 Original Filed Dec. 8, 1942 my @wm Penman 2a, 195o INnUc'rANcE con.

sianey Y. wane, amide. N. Y., mixma vim: S. Johnson, Chicago, Iii.; Victor S. Johnson, Jr., administrator de bonis non of said Victor S.

Johnson, deceased Original application December 8, 1942, Serial No.

468,195. Divided and this application November 21, 1946, Serial No. 711,437

3 Claims. (Cl. F15-21) The present application is a division of my pending application Serial No. 468,195, nled December 8, 1942, for Precision radio apparatus, now Patent No. 2,451,643. The present invention relates to important improvements disclosed in said application The complete disclosure of said application is made a part of the present specification by reference. Features of that disclosure not claimed herein are claimed in Serial No. 468,195 and in the following divisions thereof:

Ser. No. 506,372, nied'October 15, 1943, for Radio apparatus, now Patent No. 2,407,359;

Ser. No. 725,685, led Jan. 31, 1947 as a continuation of Serial No. 506,373, filed October 15, 1943, for Radio apparatus;

Ser. No. 506,374, iiled October 15, 1943, for Electrical condenser, now Patent No. 2,438,592;

Ser. No. 506,375, illed October 15, 1943, for Method of lining up uni-controlled tuned. radio apparatus, and now Patent No. 2,922,381;

Ser. No. 506,377, filed October 15, 1943, for Method of temperature compensating tuned circuits, now Patent No. 2,407,360; and

Ser. No. 711,438, led Nov. 21, 1946, for Precision radio tuner, now Patent No. 2,491,347.

It is an important object of the present invention to provide an improved inductance unit which is ailected but slightly by variations of temperature and whose response to variations of temperature is dependably maintained.

To this end, it is a salient feature that a novel combination is provided, consisting of a coil form havinga low and stable coeilicient of expansion, a tensioned conductor wound thereon to form an inductance coil, the conductor having a. relative- 1y high coeiiicient of expansion, and means for maintaining the conductor under a tension which is a substantial part of that necessary to stress the conductor to its elastic limit, whereby, in

`spite of the diilerence in expansion coefiicients ,between the conductor and the coil form, close i adhesion between the coil formv and the conl looking in the direction of the arrows;

Fig. 3 is a sectional view of the plate shown Y in Figs. 1 and 2 taken upon the line 8-8 of Fig. 2

looking in' the direction of the arrows;

Fig. 4 is an end view of a coil form employed in both the transmitter and the receiver;

Fig. 5 is a plan view of the coil form shown in Fig. 4

Fig. 6 is a longitudinal sectionalview showing the plate of Figs. 1 to 3 and the coil form of Figs. 4 and 5 in assembled relation with a coil woundv on the latter, the section being taken on the line 6 6 of Fig. 8 looking in the direction of the arrows;

Fig. 7 is a plan view of the coil assembly of Fig. 6;

Fig. 8 is a rear end view showing the coil assembly of Figs. 6 and 7 together with an assoelated tank condenser;

Fig. 9 is a view in side elevation of the coil and condenser assembly of Fig. 8;

Fig. 10 is a bottom view of the coil and condenser assembly shown in Fig. 8;

Fig. 11 is a longitudinal, sectional view showing details of the condenser of'Figs. 8 to 10, inelusive;

Fig. 12 is a rear end view of a modified form 0f coil and condenser assembly; y

Fig. 13 is a view in side elevation of the assembly of Fig. 12;

Fig. 14 is a sectional view taken upon the line lI-II of Fig. 12 looking in the direction of the arrows;

Fig. 15 is a rear end view of a further modied form of coil and condenser assembly;

Fig. 16 is a view in side elevation of the assembly of Fig. 15;

Fig. 17 is a sectional view taken upon the line |1'-|'| of Fig. 15;

Fig. 18 is a sectional view taken upon the line l8-I8 of Fig. 17 looking in the direction of the arrows; t

Fig. 19 is a sectional view similar to Fig. 17 showing a further modied form of coil;

Fig. 20 is -a rear end view of a modified form of coil and condenser assembly;

Fig. 21 is a view in side elevation of the assembly of Fig. 20; A

l Fig. 22 is a view in side elevation of a further modified form of coil and condenser assembly; and' Fig. 23 is a fragmentary4 bottom plan view illustrating a coil assembly which includes a modiiied mounting of an adjustable permeable element that forms a part of the assembly.

Figs. 1 izo-22, inclusive, disclose various desirable iilustrative forms of tuned circuit assemblies embodying the invention and parts thereof, each 'slots |88. Centrally of the block at its front and rear it is provided with arcuate shaped portions |88 and |18 from which depend the short tapered tongues |1|, and between the arcuate portions |88 and |10 the middle portion of the plate is undercut in an arcuate shape as indicated at |12.

The entire plate is iinished to the shape shown by a molding operation and is then baked at a high temperature.

Referring to Figs. 4 and 5, the coil supporting form |18 is shown as comprising a generally cylindrical shaped tube composed of the same ceramic material of which the plate |88 is formed. The coil form has a spiral shaped groove |14 ground therein adapted to accommodate the coil 88 which is herein shown as comprising a thin metallic ribbon |15 of two turns (see Fig. 6), which may be heated when applied to the coil form, so that it may develop tension through shrinkage as it cools. The coil form is also longitudinally slotted as at |18, the slot being tapered to accommodate the tongues |1| so that the slot |18 and tongues |1| provide means for locating the coil form in a deiinite position on the supporting plate |86. A material which will glaze is applied to the portions of the coil form and plate |88 which are to be brought into contact with each other and the members then baked to glaze the material which thereupon unites the support- Iing blocks and coil form into a unitary rigid stable assembly. The ceramic material is preferably of such a nature that its surface contains a large number of small particles which project beyond the general surface level and puncture the skin of the ribbon |15 in numerous places, thereby entirely preventing any slippage of the ribbon on the coil form. The result is that the coll is maintained tightly in engagement with the coil form at all times and does not change in shape due to any changes in temperature or humidity. In other words, the coil and coil form are, as it were, locked together throughout the full length of the coil and the size and shape oi the coil remain at all times the saine as those of the coil form. This arrangement obviates any non-cyclic variation in distance between one turn of the coil and another and also any non-cyclic variations in the diameter of the coil so that once the coil is wound, its inductance thereafter is not subject to non-cyclic variations due to temperature or aging. The ribbon of the coil is desirably a semi-elastic material such as sterling silver. Such material combines with high conductivity a softness permitting ready penetration by the coil form crystals and an elasticity capable of maintaining the required tension. Pure silver has been found unsuitable because it does not have the required elasticity.

Referring to Figs. '1 to 21, the powdered iron A slug I5 is secured against the lower surface of the block |88 by means of a pair of screws |11 which pass through the slots |88. The inner face |18 of the slug I8 is arcuate in shape so that it may be moved inwardly into engagement with the surface of the coil form |18. The manner of adjusting the position of the slug I5 for 4 controlling the slope of the tuningcurve of the oscillatorwill be hereinafter described. The lefthand end of the ribbon |18 is soldered to an inwardly extending tongue |18 formed on a metallic coil terminal |88 which has a nat portion |8| held against the lower face of the block |18 by a threaded hexagon head screw |82. The width of the tongue |18 is substantially equal to that of the groove |14 in the coil form so that it engages the sides of the groove and thereby prevents the coil terminal |88 from rotating when the screw v|82 is tightened up. Coll terminal I is also provided with a depending lug |88 whose lower edge is provided with an arcuate surface |84, Fig. 8, adapted to engage and be soldered to a metallic cylindrical coating or thin sleeve |85 secured to the outer peripheral surface at one end of a thin tube |88 formed of insulating material (see Figs. 9 and 11). A similar but somewhat smaller metallic coating or sleeve |81 is provided near the other end of tube 88 and the interior of the tube is provided with a thin metallic coating or sleeve |88, so that the entire unit forms an electrical condenser.

The coil terminal |88 for the other end of the coil is similar in construction to coil terminal |88, except that its parts are reversed, and corresponding parts of the two terminals are designated by the same reference numerals. The tongue |18 of terminal |88 is secured and soldered to the other end of the ribbon |18 and the arcuate surface |84 of its depending lug |88 is in engagement with and soldered to the coating |81 of the condenser. The mid-tap 58 (Fig. 9) of the coil is soldered to a tongue |88 formed on the center terminal |8| whose main body portion is fiat and is threaded to receive the securing screw |82. The tongue |88 extends substantially the full width of the spiral groove in the coil form, thus preventing rotation of coil tap |8| when the screw |82 is tightened. The upper ends of the hexagon securing screws |82 and |82 are rounded oil as indicated in Figs. 13 and 14 at |88, thereby providing switch contacts for the coil and condenser assembly. The condenser 88 is of fixed value and is connected across the ends of the oscillator coil. The securing screws |82 and |82 form switch contacts.

Referring to Figs. 17 to 19, the oscillator coil assembly shown is generally similar to that shown inFlgs. 13 to 15, the coil supporting form |18a being formed of ceramic material and being glazed to the supporting block |88 as in Fig. 13. The coil form |18a, however, is not provided with a spiral groove, but its exterior surface is cylindrical. In this case the oscillator winding 88 consists of three turns |84, |88 and |88, which are unequally spaced, the turns |88 and |88which are first entered by the core |2a being spaced closer together than the turns |85 and |84. Each coil portion consists of a thin metallic ring |81 which may be of silver or other suitable metal, and whose inside diameter is somewhat less than the outside diameter of coil form I18a. The ring |81 on one side is cut transversely, so that its ends |88 and |88 are spaced slightly apart when the coil has been slightly expanded and slipped along the coil form into position, after which its inherent resilience causes the coil to grip the outer surface of the coil form firmly and secure the ring in a dennite predetermined position on the coil form in a position normal to the axis thereof. The end |88 of turn |84 is connected to the end |88 of turn |85 by means of a metallic connector 288 which is soldered to the coil ends (see Fig. 14). A similar connector 20| connects the end |99 of turn |98 to the end |98 of turn |96, so that the current in passing through the winding flows in the same direction through all the turns. The coil terminals |80, |89 and |9| are generally similar to those shown in Figs. 8

, and 9, the terminals |80 and |89 supporting the condenser 60 and the projecting tongue |19 on the terminal |80 being soldered to the end |98 of turn |94. The tongue |19 formed on coil terminal |89 is soldered to the end |99 of turn |96,

while the tongue |90 on terminal |9| is soldered to the end |98 of turn |95, the connection of tongue |90 to turn |95 providing the intermediate tap 53 (Fig. 9) on the oscillator coil 35.

Referring to Figs. 15 to 18 another construction of winding for producing SLF is shown in which the ceramic coil form |18b is provided.

with two grooves 202 and 203 disposed normal to its axis, and the groove 208 being somewhat deeper than 202, as shown in Figs. 17 and 18. The winding has only two turns |94 and |96, each turn being formed of the metallic ribbon |15. The projecting tongue |19 of coil terminal |80 is soldered to the end |98 of turn |94 and the tongue |19 of terminal |89 is soldered to the end |99 of coil turn |96. 'I'he two turns are connected together by means of a thin metallic connector 204 which is disposed'in a longitudinally.

turns of the coil inthe same direction. The

metallic ribbon is held under tension by the connector 204, which connector is prevented from rotating by the groove 205 in the coil form so that the inner surface of the ribbon is maintained at all times in firm-engagement with the coil form. The center tap on the coil is provided by means of the tongue |90a on the center coil terminal |9I, this tongue being soldered to the middle of the connector 204 as indicated in Figs. 15 to 17. It is thus seen that the illustrated arrangement provides an oscillator coil in which the turns are of different diameter and have no pitch, and also provides a. ready means for tapping the coil at its midpoint. n

The coil arrangement shown in Fig. 19 is generally similar to that shown in Fig. 17 and the corresponding parts are indicated by similar reference numerals. In this case the connector member 204a does not bear against the sides of the groove 205 in the coil form |13c. The ends of each coil turn are provided with a pair of projecting ears 206 which abut against the wall of the groove 205 as shown. In securing the turns of ribbon to the coil form the ears 206 at one end of the ribbon are placed against the wall of groove 205 and the turn of ribbon wrapped around the coil form in the groove therein, whereupon the ribbon may be heated by an electric current from a suitable source to cause it to expand considerably in length whereupon the pair of ears 208 at the other end of the coil will be slipped into the groove 205 and the ribbon permitted to cool. The contraction of the ribbon will place it under tension and cause its interior surface to be pressed into iirm engagement with the bottom wall of the groove in which it is seated.

A modied form of oscillator coil and condenser assembly is shown in Figs. 20 and `21, the

coil being a two-turn spiral coil wound on coil form |1811, the general construction being similar to that shown in Figs. 8 to 10, but in this case the condenser 60a is of the' mica type. The tongues |19 of coil terminals |80 and |89 are soldered to the ends of the winding. and the tongue |b of middle terminal |9| is soldered to the midpoint of the winding as in Fig. 9. The mica condenser 60a comprises two metallic plates 201, 208 which are disposed against the opposite faces of a thin sheet of mica 209 which is of substantially larger area than the plates 201, 208. Coil terminal |80 is provided with an integrally formed extension 2|0 whose lower end 2|| is vertical and bears against condenser plate 201. Coil terminal |89 is provided with an integrally formed extension 2|2 whose lower portion 2|9 is vertical and bears nrmly against the plate 208 of the condenser. The condenser 80a is thus entirely supported by the extensions 2|0 and 2| 2 of coil terminals |80 and |89, these extensions being short and massive, so that their inductance is kept at a minimum value.

Spring ngers |80a and |89a are clamped between the plate |66 and the respective blocks 80 and |89 by screws |82. The ngers |80a and |89a extend toward one another and have upturned ends disposed in confronting relation. The upturned ends jointly form a conductive, spring holder for a second condenser 6|.

The coil and condenser assembly shown in Fig. 22 is similar to that shown in Fig. 21 and corresponding parts are designated by the same reierence numerals. In this case, however, the mica condenser is disposed horizontally, coil terminal |89 being provided with a flat horizontally extending portion 2|4 which bears against the upper condenser plate 201, and coil terminal |80 being provided with a iiat horizontal portion 2|5 which bears against the lower condenser plate 208 as shown. In both forms of construction no wires are required for connecting the condenser in the circuit, the connections thereto being provided by portions of the coil terminals themselves, these portions being massive and their inductance, therefore, being negligible in comparison with the inductance of the coil itself.

In all of the tuned circuit assemblies described, other than that of Figs. 12 to 14, a ribbon having a thickness of about three mils and a width of fty to seventy mils may be advantageously employed. These dimensions are cited by way of example, however, and not as dening practical limits.

At mc., if we use concentrated circuit elements of the L-C type, the coil used can be little more than 2 inches of wire. We can, therefore, use no leads whatsoever, in a disciplined circuit, as we want all the wire possible on the coil obeying a single set of rules of expansion and vibration.

Since high sustained accuracy is sought for, no structure or material can be used except of the most unchanging nature. Physically, glass, quartz and ceramic are most suitable and have good retrace characteristics of dielectric constant and physical size when varied with temperature. No structure can be employed where there is the slightest possibility of any permanent change to any degree, either electrical or mechanical.

The type of tuning employed is of the core type, and while ferrous cores will be most generally employed, the conductive type core, as for instance silver or copper, may well be used in some applications.

The tuned circuit must, therefore, be designed atoaom with the requirements of core tuning in mind. It is basic, however, that before we can tune the circuit over a range, the circuit without such tuning means must in itself maintain a nxed frequency to a high Qrder of accuracy. It must also allow trimming, tracking and aligning with a precalibrated dial having great length and accuracy of resetting. It must have no wiring at all.

The coil must allow use of one to four'turns, for example, and must allow bringing out a tap to any turn or fraction of a turn. Its external tuned circuit must include a tuning condenser having a minimum inductance, and the whole outside return loop must be minimized.

The concentration of over 90% of the inductance is actually in the coils of the tuned circuit assemblies described where it is capable of being acted on by a core.

The diameterI of the coil is chosen to be about 405 mils in the illustrative instance for use with a 375 mil core. Considerable diiiculty is had in the ceramic art in making thin walled tubes beyond a certain minimum thickness of wall. Maximum tuning ranges obtainable with core tuning are reached where the core substantialy iills up the coil, but it must still freely pass through the bore of the coil form. If we chose this same ratio with -a 125 mil core, the wall thickness would be less than mils, an impracticable figure for quantity production in the present state of the ceramic art. 1

The coil form is made with grooves for the conductor. to have the thick lands to support the thin grooves during ilring, and also to guide in the winding.

Since the conductor chosen must have high conductivity, its thermal coemcient of expansion must also be high, at least two or three times that of the coil form. A spiral winding inherently has no strength of its own, so it must be the mechanical slave of the coil form. This means the wire must be wound under sumcient tension and have enough elasticity to cling to the form at the most adverse temperature.

The cross-section of the conductor is a very thin strap, rather wide. If large, round conductors are used, such as #14 round wire, the cur rent tends to hug the coil form as it is the smallest diameter of the turn. Any good conductor has a large temperature coefilcient of resistance, however, and if the temperature be raised the resulting increased resistance causes a redistribution of the current, causing the diameter of the' mean current path to be increased. This markedly increases the inductance, since diameter of the current path is square inthe formula for the inductance coil, and great changes in frequency result.

By using a very thin strap of the order of three mils in thickness, this effect is minimized and a disciplined current path results. Instead of using pure silver, sterling silver is used for greater toughness and elasticity" and may be wound on the form quite hot by passing a heavy current through it while winding, in which case it shrinks on the form. Tension may be used also, sufficient to stress it n'early half way to its elastic limit so it hugs the coil form like a rubber band.

Silver plated alloys having negligible coeillcients of expansion, used in large cross-section, maintain their cross-sections under temperature variation, but the current redistribution is the same as for pure silver, and straps of such construction must be wound under tension and in 3 general have no advantage over the thin sterling silver strap, which may be iiattened wire.

It is of great advantage to use ceramics of the low loss type because o1 the presence on the surface oi minute sharp crystal structures which apparently pierce the skin of any unhardened metal pressed ilrmly .against them. Repeated temperature cycling oi these coils from -40 to +217 F. show no creepage of the winding, since each unit length is captured by its adjacent crystals and held firmly in place.

The length of coil chosen must also depend in part upon the tuning curve desired and upon the length of core travel most easily obtained with a desirable dial mechanism. A coil 375 mils long. measured center of winding strap to center of winding strap gives an active core movement of about 250 mils for 25% tuning range.

1n any coil to be used with a core the inside of the coil form must be lett free to pass the core. Most methods of terminating coils use rivets, eyelets. or passing the conductor through holes in the form, all of which would interfere with core movement. Some structure outside the simple cylindrical coil i'orm is, therefore, required. This takes the form of the plates or block |66 with its associated terminal blocks |80, |80 and |9| (see Figs. 13, 14 and 15).

The block |86 is preferably glazed to the coil form. Plastic cements are undesirable because of cold flow and change with age, but a good glaze in the joint red at 1700 F. really makes the two pieces unitary.

Plate |56 allows use of massive structures such as blocks |80 and |88 to be employed to give a rigid and definite termination of the inductance at either end. These blocks are given large crosssection so that they will Ihave a minimum possible inductance, and the tongues |19 provide exact termination of the inductance wound on the form, in that the take-off of the current is normal to the axis of the coil. Each -tongue |18, being the full width oi groove |14, provides a rigid non-turning structure when the contact screws |82 are tightened up. Shaping of these blocks to include the turned up portion |83 (Fig. 9) or the iiat portions 2|| and 2|3 (Fig. 21) or 2|! and 2|! (Fig. 22) allow either a cylindrical or flat type of condenser to be used for tuning the circuit.

A number of assemblies have been assembled and tested using the condenser 60 or Fig. 11, which is a commercial form where the capacity may be formed between the inner plate |88 and the two bands |81 and |05 forming two condensers in series, or the outer band |85 may be continued around the end of the hollow cylinder joining the inside plate |88 forming a single condenser through the ceramic body |88.

When this condenser is laid in the cradle formed by the connecting blocks |83 it will be seen that an absolute minimum inductance return path closing the physical separation between the ends of the coil proper has been achieved.

It is found to be a considerable advantage in this self-contained structure that rounded contacts |83 can be used as a switch in the case of multi-band apparatus. There is a real problem in switching ultra-high frequency circuits where the switch is placed within the tuned circuit.

A coil in the broadcast band may easily have ohms, which is negligible in proportion to 5000 milliohms. A two-turn coil such as shown in Fig. 13, however, may have a total RF resistance in the entire tuned circuit of only 40 milliohms. and consequently the contact resistance of any practical form of switch, which of necessity must be small because of the small physical dimensions of these circuits, becomes a substantial portion of the total resistance. It is an advantageous feature of the present invention that each coil carries its own tank condenser with it, allowing switching Voff-fthe charging current to the electrodes of the tubes only, a much easier matter.

Provision of these contacts also allows 'desirable slipping of the whole tuned circuit-assembly axially.

Provision of the plate |66 allows for the use of a solid block trimmer I as shown in Fig. 8. Plate |68 also provides a fastening means for the assembly.

The necessary provision for a tap on the coil is met by .the provision of the third contact |92 (Fig. 9), the block I9I, and the tongue |90 eni gaging the coil.

This whole assembly allows the use of variable diameter turns in the coil as shown in Fig. 16, and also variations in any desired manner of the distance between the coil turns as shown in Fig. 14.

Ihe tuned circuit assembly shown in any of the forms of Figs. 7 to 10, and 12 to 22, inclusive, makes provision for a single unit that has in effect fastening means, tuning means, switch, tank condenser, trimming, tracking' and aligning means in a single simple structure, sc that all the frequencyl determining elements are well within a cubic inch, and under temperature, vibration and shock, all travel together. There is thus provided a single universal unit that can be used for transmitter, receiver, wave trap, or any of the numerous uses to which tuned circuits can be put.

'I'he Q of these assemblies is found to be quite high without the core. If measured in air without any associated apparatus, the Q is about 700. When measured in the coil holder and with an oscillator tube assembly attached, with the tube in place but not lit, the Q exceeds 400.

A further advantage of this type of construction is that no parasitic loops of any kind are `formed to give resonant absorptive effects or resonant voltage rises at any frequency within the operating range of the current acorn tubes, and in no case below 1500 mc.

Frequency drift -due to changes in temperature have been rather fully investigated.

The true inductance of the coil varies at a rate proportional to the thermal coefficient of expansion of the coil form which is six parts per million per degree. Current redistribution due to the increased resistance of the winding conductor with heat is practically eliminated by the use of an extremely thin strap for the conductor.

Boththe real and apparent inductance cf the tuned circuit assembly lower the frequency as the temperature increases, so a slightly negative temperature coeiilcient condenser 60 is used for tuning the circuit to provide a. balance, resulting in very small change in frequency with temperature. These condensers employ generally available ceramic materials which may be chosen to yield a temperature coefilcient which is highly negative, such as an almost pure titanium dioxide mix. which gives -750 parts per million per del0 Bree, up to those going slightly positive in their temperature coemcient of capacity.

A coil stretched out over too much length isdn general undesirable as the ends are too far apart physically, forcing the use of a long return path on the outside of the coil which gives us unwanted inductance in the tunedcircuit, since only that portion of the wire in the circuit which is wound on the coil form will be affected by the core used, and any external inductance will lessen the tuning range and consequently the SLF range we may possibly reach.

It is the nature of a coil of only two or three turns spread over a length equal to its own diameter that the turns have considerable pitch, and as a consequence the flux is by no means along the axis, but is affected by the pitch of the end turn. Also, in the form of construction shown in Fig. 13 there isja mass of conductive material on one side as shown by the interconnecting blocks |80, |89 and the tuning condenser |88 which again tends to throw the flux oil' the true axis, and maximum fluxhas been observed as much as 20 off the axis when the flux distribution at the end of the coil was investigated.

An entirely different line of attack is shown by the construction illustrated in Figs. 12 and 13, where a no pitch winding is used. Here each turn, except for a. rather negligible cross-over connector, is exactly normal to the axis of the coil, and investigation showed the flux to be almost purely axial: By critical spacing of the three turns shown in Fig. 13, the rate of increase of the flux can be controlled, especially of the mutual inductance between the turns.

It has been possible to utilize this variation caused by the mutual inductance betweenthe turns to assist in the production of SLF.

It is believed to be possible to increase the tuning range by reducing the diameter of the coil form somewhat, but because of inability to procure special ceramic coil forms required it has not been possible for the present to investigate the problem further in this direction.

Where it is desired to establish tuning ranges in the portion of the spectrum above 160 megacycles or so, and still maintain a high order of circuit stability by having a good sized tuning condenser, a two-turn coil must be used. It has been found insufficient merely to vary the distance between these two turns to obtain SLF or any other desired curve, so a new variable is introduced, the choosing of different diameters of the two turns, with the turn on the entering side oi the core the smaller. The coil in which turns are of different diameters is shown in Figs. 15 to 19, inclusive.

'I'he arrangement of slug I5 shown in Figs. 8 and 10 show the slug maintained parallel to the axis of the coil. An alternative arrangement is illustrated in Fig. 23, where the slug I5 is shown pivoted at one end with the screw |62 in a close fitting round hole in the base IBSA. The other end is held with the screw |63 arranged to allow movement in the arcuate slot I 5I.

This arrangement obviates the necessity of maintaining the slug parallel to the axis of the coil during the adjustment of the slug and is found somewhat more effective in some cases in displacing the tuning curve a large amount at the lower frequency end of the tuning range while minimizing any unwanted eects at the high frequency end thereof.

Where it is desired to precisely adjust the resonant frequency of an oscillator or other resonant circuit to a single predetermined frequency, the capacity and inductive elements of the circuit may be made to tune to this frequency as close as practical considerations permit, leaving the precise adjustment to the correct frequency to the manipulation of the slug l5. It will, therefore, be seen that a single self-contained unit in the absence of the movable core I2a permits radio apparatus to be built to resonate at a single assigned frequency in a very simple, compact and stable form.

I have described what I believe to be the best embodiments of my invention. I do not wish, however, to be confined to the embodiments shown, but what I desire to cover by Letters Patent is set forth in the appended claims.

I claim:

1. In combination, a rigid cylindrical, ceramic coil form having a very low, stable coeilicient of expansion, an inductance in the form of a flat, highly conductive, semi-elastic ribbon supported thereon, the ribbon having a relatively high coemcient of expansion, and means for anchoring the ribbon to the coil form and for maintaining the ribbon under a tension which is a substantial part of that necessary to stress the ribbon to its elastic limit, the coil form having a surface structure of fine, sharp, crystalline form which penetrates the ribbon surface and interlocks with it to exert a snubbing action, the construction and arrangementbeing such that, notwithstanding the difference in expansion coeillcients between the ribbon and the coil form, close adhesion of the ribbon to the coil form is maintained by maintaining the tension of the conductor throughout the operative temperature range.

2. In combination, a rigid cylindrical coil form, an inductance in the form of a flat, highly conductive, semi-elastic ribbon mounted thereon, the ribbon having a relatively high coefficient of expansion, and means for anchoring the ribbon to the coil form and for maintaining the ribbon under a tension which is a substantial part of that necessary to stress the ribbon to its elastic limit, the coil form being composed of insulating ceramic material which has a low coefficient of expansion relative to the ribbon and is unaffected in its physical and electrical properties by changes of temperature within the operative temperature range, and which has a surface structure of fine, sharp, crystalline form which penetrates the ribbon surface and interlocks with it to exert a snubbing action, the construction and arrangement being such that, notwithstanding the difference in expansion coefficients between the ribbon and the coil form, close adhesion of the ribbon to the coil form is maintained by maintaining the tension of the conductor throughout the operative temperature range.

3. In combination, a rigid cylindrical coil form, a tensioned conductor of uniform cross-section disposed around the outside of the coil form to form an inductance coil, and means for anchoring the conductor to the coil form and for maintaining the conductor in tensioned engagement with the coil form, the coil form being composed of insulating material whose changes in physical and electrical characteristics with temperature are small, and of great cyclical and secular stability, and which has a structure of fine, sharp, crystalline form which penetrates the conductor surface and interlocks with it to exert a snubbing action, the construction and arrangement being such that, notwithstanding the difference in expansion coeilicients between the conductor and the coil form, close adhesion of the conductor to the coil form is maintained by maintaining the tension of the conductor throughout the normal temperature range.

SIDNEY Y. WHITE.

REFERENCES orrnn The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,891,481 Scofield Dec. 20, 1932 1,897,253 Gaubert et al Feb. 14, 1933 1,904,199 Brewer Apr. 18, 1933 1,910,957 Llewellyn May 23, 1933 1,971,452 Herrmann Aug. 28, 1934 1,996,823 Osnos Apr. 9, 1935 2,018,626 Polydoroif Oct. 22, 1935 2,115,826 Norton et al May 3, 1938 2,268,742 Daugherty Jan. 6, 1942 2,416,393 Huckleberry Feb. 25, 1947 FOREIGN PATENTS Number Country Date 624,391 Germany Jan. 18, 1936 OTHER REFERENCES Article, The Temperature Coefilcient of Inductances for Use in a Valve Generator, by E. B. Moulin, from Proceedings of the Institute of Radio Engineers, vol. 26, No. 11, Nov. 1938. 

